This invention relates generally to radar systems, and more specifically to a radar system which is capable of synchronization with a digital elevation map (DEM) to accurately determine a location.
The proper navigation of an aircraft in all phases of its flight is based to a large extent upon the ability to determine the terrain and position over which the aircraft is passing. In this regard, instrumentation, such as radar systems, and altimeters in combination with the use of accurate electronic terrain maps, which provide the height of objects on a map, aid in the flight path of the aircraft. Electronic terrain maps are well known and are presently used to assist in the navigation of aircraft.
Pulse radar altimeters demonstrate superior altitude accuracy due to their inherent leading edge return signal tracking capability. The pulse radar altimeter transmits a pulse of radio frequency (RF) energy, and a return echo is received and tracked using a tracking system. The interval of time between signal bursts of a radar system is called the pulse repetition interval (PRI). The frequency of bursts is called the pulse repetition frequency (PRF) and is the reciprocal of PRI.
FIG. 1 shows an aircraft 2 with the Doppler effect illustrated by isodops as a result of selection by the use of Doppler filters. The area between the isodops of the Doppler configuration will be referred to as swaths. The Doppler filter, and resulting isodops are well known in this area of technology and will not be explained in any further detail. Further, the aircraft 2 in the specification will be assumed to have a vertical velocity of zero. As is known, if a vertical velocity exists, the median 8 of the Doppler effect will shift depending on the vertical velocity. If the aircraft 2 has a vertical velocity in a downward direction, the median of the Doppler would shift to the right of the figure. If the aircraft 2 has a vertical velocity in an upward direction, the Doppler would shift to the left of the figure. Again, it will be assumed in the entirety of the specification that the vertical velocity is zero for the ease of description. However, it is known that a vertical velocity almost always exists.
Radar illuminates a ground patch bounded by the antenna beam 10 from an aircraft 2. FIG. 1a shows a top view of the beam 10 along with the Doppler effect and FIG. 1b shows the transmission of the beam 10 from a side view. To scan a particular area, range gates are used to further partition the swath created by the Doppler filter. To scan a certain Doppler swath, many radar range gates operate in parallel. With the range to each partitioned area determined, a record is generated representing the contour of the terrain below the flight path. The electronic maps are used with the contour recording to determine the aircraft""s position on the electronic map. This system is extremely complex with all the components involved as well as the number of multiple range gates that are required to cover a terrain area. As a result, the computations required for this system are very extensive.
In addition to the complexity, the precision and accuracy of the distance to a particular ground area or object has never been attained using an airborne radar processor.
In one aspect, an in-phase/quadrature component (IQ) mixer is provided. The mixer is configured to reject returns from a negative doppler shift swath in order to mitigate corruption of a positive doppler shift swath. The mixer comprises a sample delay element configured to produce a quadrature component, a plurality of mixer elements, a plurality of low pass filters electrically connected to outputs of the mixer elements, a plurality of decimators electrically connected to outputs of the low pass filters, a plurality of all pass filters electrically connected to outputs of the decimators, and a subtraction element electrically connected to outputs of the all pass filters. The mixer is configured to sample and filter both an in-phase component and a quadrature component of a received signal.
In another aspect, a method for processing radar return data is provided. The method allows rejection of radar returns from a negative doppler shift swath in order to mitigate corruption of radar returns from a positive doppler shift swath. The radar is configured to receive returns at each of a right channel, a left channel, and an ambiguous channel. The method comprises sampling the radar data from each of the channels, filtering the samples, converting the filtered samples to a doppler frequency, filtering the doppler frequency signals with a band pass filter, the filter centered at the doppler frequency, and determining a phase relationship between the right, left, and ambiguous channels using the filtered doppler frequency signals.
In still another aspect, a radar signal processing circuit is provided. The processing circuit comprises a radar gate correlator configured to sample radar data at a sampling rate, a correlation bass pass filter configured to stretch the sampled radar data to a continuous wave (CW) signal, and a mixer configured to generate a quadrature component of the CW signal using a sample delay element. The mixer is further configured to down sample an in-phase component and the quadrature component of the CW signal to a doppler frequency. The processing circuit further comprises a band pass filter centered on the doppler frequency.